Hi guys & gals, I'm hoping someone would be able to help with an issue with the belt tensioner/pulley. I developed a pretty loud knock from the engine at low revs (idle - below 1000) that gradually disappears around the 1200 mark. My regular Benz mechanic pinpointed the source to the belt tensioner and was confirmed by removing the belt. It was defiantely coming from there, as the knock could be physically felt from a long screwdriver placed on it.

I waited a couple of days for the specific pulley to come in (apparently there are a few for the S320's (!), but even with the new one, the knocking persists. We changed the pulley first, which made no difference followed by the actual tensioner unit. We also tried a second hand one from an engine pulled from another car, and still no difference. It's pretty loud now (and very annoying). No symptoms - car runs real nice as always, temp/pressure/oil quantity is good.

He's a great mechanic - but granted he doesn't get many 140's in - and this is the first time in 12 years I've ever seen him scratch his head! Really hoping someone can lend a hand with this one....

Cheers Peter - Australia

 

how do you know its not the water pump?

 

I don't know if it's the water pump or not. The actual knocking is definately coming from that belt tensioner. Is it possible for the water pump to crap itself (and harder for the pulley to spin) then somehow transfer the knocking to the belt tensioner pulley?